This invention relates to an apparatus for detecting the passage of superposed sheets along a feed path. The invention has application, for example, to an apparatus for detecting the passage of superposed currency notes in an automated teller machine (ATM).
In a cash dispensing mechanism of an ATM, it is important to provide a simple and reliable means for detecting when a currency note has become superposed on another in a path of travel from a currency supply means to a note exit slot, since such superpositioning may produce an undesirable result such as the dispensing of an excessive amount of money. For convenience, two or more sheets or notes which have become superposed will hereinafter be referred to as a multiple sheet or a multiple note.
A known system for detecting multiple sheets includes a sensing station in a feed path for the sheets. The sensing station includes a light emitter and an optical sensor arranged to sense light from the emitter transmitted through an item comprising a single or multiple sheet present at the sensing station. The optical sensor provides an output voltage whose magnitude is dependent on the intensity of the transmitted light received by the optical sensor. The output voltage of the optical sensor is applied to a processing circuit, and on the basis of this voltage the processing circuit provides an output indicative of whether a single sheet or a multiple sheet is present at the sensing station.
With this known system problems have been experienced in reliably distinguishing between a single bank note and a double bank note.
Referring to FIG. 1, the output of an optical sensor as a result of light incident on it varies exponentially with changes in transmissivity across a gap between a light emitter and the optical sensor. Point A on the graph represents the sensor output when there is no note present between the sensor and the emitter, point B represents the sensor output when there is one note present between the sensor and the emitter, and point C represents the sensor output when there are two notes present between the sensor and the emitter. It is seen that there is a relatively large difference between the sensor outputs for when there is no note present and when there is one note present, but that there is a relatively small difference between the sensor outputs for when there is one note present and when there are two notes present. Thus, there is not a very clearly defined difference as regards the sensor output for single and multiple notes. Also, the very low sensor output for a multiple note may approach the noise level of an analog-to-digital (A/D) converter included in the circuit used to process the output of the sensor.
Another known system for detecting multiple sheets is disclosed, for example, in EP-B-0344938 (Proprietor: NCR International, Inc.). This system includes first and second co-operating rollers between which sheets pass as they are fed along a feed path, the first roller having a fixed axis of rotation, and the second roller being resiliently urged towards the first roller so as to enable it to be moved away from the first roller as a single or multiple sheet passes between the rollers. A voltage generating means associated with the second roller produces an output voltage which varies linearly with movement of the second roller towards or away from the first roller, and this output voltage is applied to a processing circuit which determines whether a single or multiple sheet has passed between the rollers. This known system has the disadvantage that it is relatively complicated compared with the known multiple sheet detector system using an optical sensor.